From b4382b7055fc8b0078cbb50ed9c3f924635d9971 Mon Sep 17 00:00:00 2001
From: Bas Peschier <bpeschier@bpeschier.nl>
Date: Sat, 21 Mar 2015 13:19:13 +0100
Subject: [PATCH] Fixed #16362 -- Allowed lookaround assertions in URL
 patterns.

---
 django/utils/regex_helper.py       | 11 ++++---
 docs/releases/1.9.txt              |  5 ++++
 docs/spelling_wordlist             |  1 +
 tests/urlpatterns_reverse/tests.py | 47 ++++++++++++++++++++++++++++++
 tests/urlpatterns_reverse/urls.py  |  4 +++
 5 files changed, 62 insertions(+), 6 deletions(-)

diff --git a/django/utils/regex_helper.py b/django/utils/regex_helper.py
index 53736dffa7..4a697b2920 100644
--- a/django/utils/regex_helper.py
+++ b/django/utils/regex_helper.py
@@ -59,11 +59,10 @@ def normalize(pattern):
     (3) Select the first (essentially an arbitrary) element from any character
         class. Select an arbitrary character for any unordered class (e.g. '.'
         or '\w') in the pattern.
-    (5) Ignore comments and any of the reg-exp flags that won't change
-        what we construct ("iLmsu"). "(?x)" is an error, however.
-    (6) Raise an error on all other non-capturing (?...) forms (e.g.
-        look-ahead and look-behind matches) and any disjunctive ('|')
-        constructs.
+    (4) Ignore comments, look-ahead and look-behind assertions, and any of the
+        reg-exp flags that won't change what we construct ("iLmsu"). "(?x)" is
+        an error, however.
+    (5) Raise an error on any disjunctive ('|') constructs.
 
     Django's URLs for forward resolving are either all positional arguments or
     all keyword arguments. That is assumed here, as well. Although reverse
@@ -129,7 +128,7 @@ def normalize(pattern):
                     walk_to_end(ch, pattern_iter)
                 else:
                     ch, escaped = next(pattern_iter)
-                    if ch in "iLmsu#":
+                    if ch in "iLmsu#!=<":
                         # All of these are ignorable. Walk to the end of the
                         # group.
                         walk_to_end(ch, pattern_iter)
diff --git a/docs/releases/1.9.txt b/docs/releases/1.9.txt
index 3fa068c97f..10a4db5518 100644
--- a/docs/releases/1.9.txt
+++ b/docs/releases/1.9.txt
@@ -205,6 +205,11 @@ Tests
 
 * ...
 
+URLs
+^^^^
+
+* Regular expression lookaround assertions are now allowed in URL patterns.
+
 Validators
 ^^^^^^^^^^
 
diff --git a/docs/spelling_wordlist b/docs/spelling_wordlist
index 78480713fb..cf9a71db97 100644
--- a/docs/spelling_wordlist
+++ b/docs/spelling_wordlist
@@ -436,6 +436,7 @@ Login
 logout
 Logout
 Loïc
+lookaround
 lookup
 Lookup
 lookups
diff --git a/tests/urlpatterns_reverse/tests.py b/tests/urlpatterns_reverse/tests.py
index eca5bc1e64..8706c64b64 100644
--- a/tests/urlpatterns_reverse/tests.py
+++ b/tests/urlpatterns_reverse/tests.py
@@ -799,3 +799,50 @@ class IncludeTests(SimpleTestCase):
         msg = "Must specify a namespace if specifying app_name."
         with self.assertRaisesMessage(ValueError, msg):
             include('urls', app_name='bar')
+
+
+@override_settings(ROOT_URLCONF='urlpatterns_reverse.urls')
+class LookaheadTests(TestCase):
+    def test_valid_resolve(self):
+        test_urls = [
+            '/lookahead-/a-city/',
+            '/lookbehind-/a-city/',
+            '/lookahead+/a-city/',
+            '/lookbehind+/a-city/',
+        ]
+        for test_url in test_urls:
+            match = resolve(test_url)
+            self.assertEqual(match.kwargs, {'city': 'a-city'})
+
+    def test_invalid_resolve(self):
+        test_urls = [
+            '/lookahead-/not-a-city/',
+            '/lookbehind-/not-a-city/',
+            '/lookahead+/other-city/',
+            '/lookbehind+/other-city/',
+        ]
+        for test_url in test_urls:
+            with self.assertRaises(Resolver404):
+                resolve(test_url)
+
+    def test_valid_reverse(self):
+        url = reverse('lookahead-positive', kwargs={'city': 'a-city'})
+        self.assertEqual(url, '/lookahead+/a-city/')
+        url = reverse('lookahead-negative', kwargs={'city': 'a-city'})
+        self.assertEqual(url, '/lookahead-/a-city/')
+
+        url = reverse('lookbehind-positive', kwargs={'city': 'a-city'})
+        self.assertEqual(url, '/lookbehind+/a-city/')
+        url = reverse('lookbehind-negative', kwargs={'city': 'a-city'})
+        self.assertEqual(url, '/lookbehind-/a-city/')
+
+    def test_invalid_reverse(self):
+        with self.assertRaises(NoReverseMatch):
+            reverse('lookahead-positive', kwargs={'city': 'other-city'})
+        with self.assertRaises(NoReverseMatch):
+            reverse('lookahead-negative', kwargs={'city': 'not-a-city'})
+
+        with self.assertRaises(NoReverseMatch):
+            reverse('lookbehind-positive', kwargs={'city': 'other-city'})
+        with self.assertRaises(NoReverseMatch):
+            reverse('lookbehind-negative', kwargs={'city': 'not-a-city'})
diff --git a/tests/urlpatterns_reverse/urls.py b/tests/urlpatterns_reverse/urls.py
index d6b692d220..25609c14cf 100644
--- a/tests/urlpatterns_reverse/urls.py
+++ b/tests/urlpatterns_reverse/urls.py
@@ -62,6 +62,10 @@ with warnings.catch_warnings():
         url(r'^outer/(?P<outer>[0-9]+)/', include('urlpatterns_reverse.included_urls')),
         url(r'^outer-no-kwargs/([0-9]+)/', include('urlpatterns_reverse.included_no_kwargs_urls')),
         url('', include('urlpatterns_reverse.extra_urls')),
+        url(r'^lookahead-/(?!not-a-city)(?P<city>[^/]+)/$', empty_view, name='lookahead-negative'),
+        url(r'^lookahead\+/(?=a-city)(?P<city>[^/]+)/$', empty_view, name='lookahead-positive'),
+        url(r'^lookbehind-/(?P<city>[^/]+)(?<!not-a-city)/$', empty_view, name='lookbehind-negative'),
+        url(r'^lookbehind\+/(?P<city>[^/]+)(?<=a-city)/$', empty_view, name='lookbehind-positive'),
 
         # Partials should be fine.
         url(r'^partial/', empty_view_partial, name="partial"),